Outline
- Links
- Cables
- Leased Lines
- Last-Mile Links
- Wireless Links
Introduction
- On Monday we began talking about Direct Link Networks.
- We looked at the hardware involved in network nodes.
- We discussed how the CPU communicates with the network
adapter using the CSR and can send and receive using either DMA or PIO.
- We made the point that CPU speeds are increasing faster than memory
access speeds, as well as bus speeds, and this can limit our ability to send data as fast as we might want.
- Also, it shows we don't want to perform too many reads or write to memory in transferring data.
- Another issue we discussed is that our memory is finite and so this limits our ability to buffer on a given machine.
- Today, we are going to look at the hardware that implements links.
Links
- Network links are implemented on a variety of physical media: twisted pair, optical fiber, coaxial cable, or using radio, microwave, or infrared.
- Usually, the signal being sent is some kind of electromagnetic wave.
- One important property of such waves are their frequency -- the speed at which they oscillate.
- The inverse of the frequency -- the wavelength -- of the wave is the distance between adjacent maxima or minima of the wave. It is usually measured in meters.
- More precisely, frequency and wavelength are related by f λ=c.
- The next slide has a diagram illustrating common frequencies for different propagation media.
The EM Spectrum
From Waves to Bits and Channels
- So far we have described how a physical medium carries signals in the form of waves...
- We often want to transmit digital. information like 0's and 1's. How we piggy-back binary data
on waves is called an encoding
- It has two layers:
- A lower layer which is concerned about modulation -- varying the frequency, amplitude, or
phase of the signal to effect transmission of information. For example, one might vary the power (amplitude) of a single wavelength between two values -- the "high" signal and the "low" signal.
- An upper layer which is concerned with encoding bits using these two different signal values. (What we'll be interested in for this class.)
- Another issue about a links is how many bit-streams can be encoded on a link at a given time. If only one, we have to have a
mechanism for sharing access such as what we will describe for multiple-access links. For point-to-point links, if the answer is two, then one signal can be used to communicate from Host A to Host B and the other for the way back. This is called a
full-duplex link. If the answer is one then Host A and B must take turns and the channel is called
half-duplex.
Cables
- One way to connect two machines is to physically string a cable between the two.
- We next consider a couple of different cable types...
Twisted Pair
- Twisted pair wire consists of two insulated copper wires
typically about 1mm thick which are twisted together.
- As a charge travels down a wire it gives off EM radiation,
which is power lost.
- Twisting causes the waves that are radiated to cancel
preventing the wires from radiating as effectively.
- Most telephones are connected to the phone company by
twisted pair wires.
- A signal can be sent down such a wire without
amplification for several kilometers. For longer distances a
repeater is necessary.
- The bandwidth of these wires is on the order of
megabits/sec, and they are very cheap.
- The most common kind of twisted pair wires used in networks
today is some flavor of Category 5 (Cat-5). More recently, Cat-5e might be used for
gigabit networks.
Coaxial Cable
- This kind of wire has better shielding then twisted pair
wires, so it can span wider distances at higher speeds.
- It is often called coax.
- There are two types of coax cables 50 and 75 ohm.
- 50 ohm was intended for digital transmission; whereas, 75
ohm was intended for both digital as well as analog
tramsmission such as TV.
- Modern cables are capable of bandwidth around 1GHz.
- In addition to cable TV use, these cables used to be used
by the phone company over long distances before fiber
optics became more popular.
Fiber Optics
- This is the largest bandwidth transmission technology.
- Currently systems can operate at 10Gbps, with lab
examples running at 100Gbps over a single fibre.
- Optical transmission has three components: the light
source, the medium, and the detector.
- A pulse of light is used to indicate a 1, the absence of a
pulse a 0.
- The transmission medium is an ultra-thin fibre of glass.
- A detector generates electrical pulses when light fall on it.
- Light does not leak out of the fiber because it get reflect
off the sides of the fibre back into the fibre
- Angles of incident light at which this ricochet effect will
work are called modes of the fiber.
- You can have single as well as multi-mode fiber.
A Table Comparing Common Cables and Fibers
| Cable | Typical Bandwidths | Distance |
|---|
| Cat-5 Twisted Pair | 10-100Mbps | 100m |
| Thin-net Coax | 10-100Mbps | 200m |
| Thick-net Coax | 10-100Mbps | 500m |
| Multimode Fiber | 100Mbps | 2km |
| Single Mode Fiber | .1-10 Gbps | 40km |
Leased-Lines
- If the two nodes you want to connect are on the opposite sides of the country, or even across town, it is not
practical to install the link yourself.
- You could lease such a dedicated link from the phone company called a "leased line".
- Below is a table of the various "leased line" services the phone company might provide:
| Service | Bandwidth |
|---|
| DS-1 | 1.544 Mbps |
| DS-3 | 44.736 Mbps |
| STS-1 | 51.840 Mbps |
| STS-3 | 155.250 Mbps |
| STS-12 | 622.080 Mbps |
| STS-24 | 1.244160 Gbps |
| STS-48 | 2.488320 Gbps |
More on Leased-Lines
- DS1 and DS3 (sometimes called T1 and T3, DS := digital signal) are older technologies originally defined for copper-based
transmission media.
- DS1 is equal to the aggregation of 24 digital voice circuits of 64kbps each and DS3 is equal to an aggregration
of 28 DS1 links.
- All the STS-N links are optical fiber (STS stands for Synchronous Transport Signal).
- STS-1 is the base link speed and STS-N has N times the bandwidth of STS-1.
- STS-N is sometimes called OC-N (optical carrier).
- Typically, the phone company does not implement a leased link using a single cable, but instead uses its own internal network.
Last-mile Links
- Instead of a dedicated leased line, it is often cheaper to connect two hosts by connecting each of them to a network using
a last-mile link.
- Common last-mile services are: modem (POTS - plain old telephone service), Integrated Services Digital Network (ISDN) - two 64Kbps channels sticks together one for voice the other for data (can ditch voice if want), xDSL (some flavor of digital subscriber line ADSL or VDSL) goes over POTS but where the voice filter on local loop has been removed at end office, and cable modem.
- Below is a table of common last-mile links and their bandwidths:
| Service | Bandwidths |
|---|
| POTS | 28.8-56Kbps |
| ISDN | 64-128Kbps |
| xDSL | 128Kbps-100Mbps |
| CATV | 1-40Mbps |
Wireless Links
- Wireless links transmit electromechanical signals - radio, microwave, infrared, or even optical light (optical telegraphy was used by Napoleon).
- The space through which these signals are transmitted is shared.
- Since signal strength falls off with the square of the distance, a given source attenuates with distance and this can be used to limit who hears who in space.
- The other way we can control who uses this common resource is by coming up with protocols about which frequency to send data on.
- In the U.S., the FCC (Federal Communications Commission) is responsible for reserving certain frequency bands for certain uses: AM radio, FM radio, TV, etc.
- It also licenses specific frequencies within these bands to organizations for a given area. For example, KQED.
- There are license-exempt frequencies that have different restrictions on their use: low power, etc.
- There are several techniques for sharing a limited spectrum of frequencies among many users so as to reduce interference (spread spectrum techniques):
- frequency hopping -- sender and receiver share a pseudo-random number generator (prng) seed, and use this seed to compute
a sequence of frequencies to briefly communicate on.
- direct-sequence -- each bit of data is represented by multiple bits in the transmitted signal, so if some bits are damaged by interference
it is often still possible to figure out if the code was for a 1 or a 0. For each bit the sender wants to transmit, the sender actually
XORs that bit with n random bits generated by a (prng) known to the sender and receiver. The transmitted value is known as an n-bit chipping code.
- Spread spectrum techniques are used with 802.11 (Wi-fi), 802.15.1 (Bluetooth), 802.16(Wi-Max).
